A person who swears an oath or makes a vow together with another person.
From Latin 'co-' meaning 'together' and 'swearer' from Old English 'swerian' meaning 'to make an oath'. The compound indicates someone bound by the same sworn promise.
In medieval oaths and pacts, 'coswearer' captured something legally and socially important—multiple people swearing together created a bond that bound all of them equally, making any betrayal a violation by everyone.
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